I mess with Unity not Unreal, but... while having the fastest machine possible speeds up your Debug cycle, not having the fastest machine possible lets you know as soon as you are building something that won't run efficiently on lower spec hardware. i.e. you avoid needing to rewrite everything at the end.
This is a double edged sword; recently I was running a test scene with a couple of hundred npcs and I hit performance limits, so I had to completely rebuild how they work, much earlier than I would have liked to, but on the other hand I now have fleshed out quite a bit of the npc "ai" and its made me change some earlier decisions for the better. Making those changes at the end would be way more costly.